The present invention relates to the transmission of information in digital form over fading channels.
The increasing prominence of wireless telecommunications--particularly in the realm of digital cellular mobile radio--has given rise to the demand for improvements in the bandwidth efficiency of such systems. To this end, efficient coded modulation schemes, such as the built-in time-diversity technique disclosed in U.S. Pat. No. 5,029,185, issued to L. F. Wei on Jul. 2, 1991 and entitled "Coded Modulation for Mobile Radio," have been developed. Increased bandwidth efficiency is also achieved in these systems via the use of low-bit-rate speech coders, such as the so-called code-excited linear predictive (CELP) coders, which operate in the range of about 4 to 8 kilobits per second (kbps).
The low-bit-rate coders represent speech information in such a way that certain aspects of the coded information is of significantly greater importance than other aspects in terms of being able to recover intelligible speech at the receiver. In CELP coders, for example, that so-called "important information" may comprise a) the linear predictive coding (LPC) parameters, b) the pitch, and c) a bit of information which indicates whether the speech was voiced or unvoiced. It is thus desirable that the transmission scheme be able to communicate the "important information" with a high degree of reliability, even at the expense--if channel conditions make it necessary--of the other, "less important" information. Such transmission schemes are referred to herein generically as schemes which provide "unequal error protection." And it should also be noted at this point that, in general, there can be any desired number of classes of information, of varying importance, rather than being limited to, for example, two classes.
Transmission schemes which provide unequal error protection are, in fact, known in the prior art. Such known technology is exemplified by, for example, in Carl-Erik Sundberg, "Optimum Weighted PCM for Speech Signals," IEEE Transactions on Communications, Vol. COM-26, No. 6, June 1978, pp. 872-881; C.-E. W. Sundberg et al., "Logarithmic PCM weighted QAM transmission over Gaussian and Rayleigh fading channels," IEE Proceedings, Vol. 134, Pt. F, No. 6, October 1987, pp. 557-570; and in the commonly assigned co-pending patent applications of V. B. Lawrence et al., Ser. No. 07/611,225 filed Nov. 7, 1990 (now U.S. Pat. No. 5,164,963, issued Nov. 17, 1992) and entitled "Coding for Digital Transmission," and of L. F. Wei, Ser. No. 07/611,200 filed Nov. 7, 1990 (now U.S. Pat. No. 5,105,442, issued Apr. 14, 1992) and entitled "Coded Modulation with Unequal Error Protection." Wireless telecommunications of the type that the present invention is concerned with are typically carried out over so-called fading channels, by which is meant so-called Rayleigh or near-Rayleigh channels, where multiplicative Rayleigh noise is the predominant noise phenomenon. However, the prior art (for coded modulation) has generally addressed the unequal error protection problem in the context of transmission of the information over, for example, voiceband telephone channels and HDTV channels, where additive white Gaussian noise is the predominant noise phenomenon, and those schemes will not perform effectively if used for the fading environment. Moreover, although aforementioned coded modulation schemes as disclosed in the Wei patent application are, in fact, directed to fading channel applications, they provide equal error protection for the transmitted data rather than the unequal error protection that is so highly desirable for the aforementioned coded speech applications. It can also be noted that schemes which provide unequal error protection in conjunction with binary and quaternary phase shift keyed signals are known in the prior art. See "Rate-compatible punctured convolutional codes for digital mobile radio," by J. Hagenauer, N. Seshadri and C.-E. W. Sundberg in IEEE Transactions on Communications, Vol. 37, No. 7, July 1990. However, these schemes are limited to a maximum of two bits per symbol, irrespective of the signal-to-noise ratio (SNR) of the channel and are thus bandwidth-inefficient. Moreover, even when achieving two bits per symbol, these schemes are power-inefficient.
There thus remains in the art the need for effective, bandwidth- and power-efficient transmission schemes which can provide unequal error protection in fading channel environments.